Various electromechanical systems for controlling the operation of motor vehicle turn signal and hazard flasher systems are known and widely used in the automotive and related industries. Some such systems include a single output control module that distributes a flashing signal to the various lamps of the vehicle according to a selected signal function. These types of systems typically include a number of switches integrated into the steering column, whereby the switches are appropriately controlled to distribute the flashing signal to one or more of the vehicle lamps. Other known turn signal and hazard lamp control systems include a multiple output control module. These types of modules typically include multiple inputs for receiving the various turn signal and/or hazard signal requests, whereby the control module is operable to direct the selected signal function to the appropriate lamp output or outputs.
Regardless of the actual configuration of the turn signal and hazard flasher control system, vehicle lamp configurations are typically described in terms of the number of lamps required to be illuminated during turn and/or hazard modes of operation. Vehicle lamp arrangements are thus typically denoted as "x:y", wherein the first number, x, refers to the number of lamps illuminated during turn signal operation and the second number, y, refers to the number of lamps illuminated during hazard mode. For example, many passenger vehicle applications are denoted as "2:4" since two lamps are typically illuminated during turn mode and four lamps are illuminated during hazard mode. Other passenger vehicle applications as well as standard passenger vehicle applications having a trailer or other towed vehicle attached thereto are denoted as "3:6" since three lamps are typically illuminated during turn mode and six lamps are illuminated during hazard mode.
Pertinent U.S. and European legal regulations require detection and driver notification of failure of one or more of the turn signal lamps. European legal regulations further require detection and driver notification of failure of one or more turn signal lamps of an attached trailer or other towed vehicle, although the U.S. regulations currently have no such requirement. In either case, one known technique for detecting failure of a turn signal lamp is to measure lamp current via one or more shunt resistors in series with one or more of the vehicle lamps. Upon detection of a drop in current due to one or more lamp failures, one common form of providing a driver warning is to change the frequency of the flash signal, typically by doubling the lamp flash rate.
One drawback associated with the use of one or more shunt resistors in detecting turn signal lamp failures is that the threshold current for detection of one or more lamp failures depends upon the number and wattages of the various lamps in the turn signal and hazard flasher system. Thus, for example, a 2:4 system having a single shunt resistor through which all lamp current passes has a different failure current threshold than the same shunt resistor in a 3:6 system. Accordingly, a shunt-based turn signal lamp failure detection system designed for a particular number of turn signal lamps having specific wattages will not work properly if a trailer is attached to the vehicle and/or if the number/wattages of the various turn signal lamps in the vehicle are changed.
One known technique for overcoming the foregoing drawback is disclosed in U.S. Pat. No. 5,805,061 to Fritz et al. The Fritz et al. system includes a memory circuit having a load failure threshold voltage stored therein and circuitry for monitoring the voltage across a shunt resistor and comparing the shunt resistor voltage to the stored load failure threshold voltage. If the shunt resistor voltage is lower than the stored load failure threshold voltage, a lamp failure is detected and the turn signal/hazard flashing rate is doubled. If, on the other hand, the shunt resistor voltage is higher than the stored load failure threshold voltage, this indicates that additional lamps have been added to the system (e.g., a trailer has been attached to the vehicle) and/or the wattages of the existing turn signal lamps have been increased. In this case, a second higher load failure threshold voltage, corresponding to the newly detected higher shunt resistor voltage, is stored in memory and the measured shunt resistor voltage is thereafter compared to the second load failure threshold voltage for monitoring of turn signal lamp failures.
While the Fritz et al. reference appears to address the problem with shunt-based lamp failure detection systems described hereinabove, it has several drawbacks associated therewith. For example, the Fritz et al. circuitry requires a memory unit and a number of comparator circuits in addition to the conventional flasher circuitry, and is accordingly unnecessarily complex. Further, the Fritz et al. system does not appear to account for the possibility that additionally connected lamps may be failed prior to connection. The Fritz et al. system accordingly may not be able to detect operational failures associated with pre-failed lamps.
What is therefore needed is an improved shunt-based turn signal and hazard flasher control system that is operable to detect lamp failures in a number of lamp configuration systems. Such a control system should preferably be reliable, simple in construction and operable to detect operational as well as preexisting lamp failures.